Extract shared BackLink and adopt on detail pages

The workout, values, score, and medication detail pages each hand-rolled
the same ghost Button + Link + ArrowLeft back-nav block. Fold it into a
single BackLink (href + label + optional data-slot) so the affordance
reads identically — same hit area, icon, and left-bleed alignment — on
every page and lives in one place. Each page passes its existing label
translation, so no new i18n keys are introduced and the markup stays
byte-identical.

src/components/ui/__tests__/back-link.test.tsx

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+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { BackLink } from "../back-link";
+
+describe("<BackLink>", () => {
+  it("renders an anchor to the href carrying the label", () => {
+    const html = renderToStaticMarkup(
+      <BackLink href="/insights/workouts" label="Back to workouts" />,
+    );
+    expect(html).toContain('href="/insights/workouts"');
+    expect(html).toContain("Back to workouts");
+    // The decorative chevron is hidden from screen readers; the label
+    // carries the meaning.
+    expect(html).toContain('aria-hidden="true"');
+  });
+
+  it("keeps the shared left-bleed alignment on the control", () => {
+    const html = renderToStaticMarkup(
+      <BackLink href="/medications" label="Back" />,
+    );
+    // The `-ml-2 w-fit` left-bleed is the standard back-nav placement; it
+    // must survive on every adopting page.
+    expect(html).toContain("-ml-2");
+    expect(html).toContain("w-fit");
+  });
+
+  it("threads the optional data-slot for e2e/visual targeting", () => {
+    const html = renderToStaticMarkup(
+      <BackLink
+        href="/insights"
+        label="Back"
+        dataSlot="composite-score-back"
+      />,
+    );
+    expect(html).toContain('data-slot="composite-score-back"');
+  });
+});